Color television demodulation system



Oct. 8; 1968 N, w. PARKER 2 3,405,230

COLOR TELEVISION DEMODULATION SYSTEM Filed Feb. 12, 1968 2 Sheets-Sheet1 l4 l5 r E FIGJ SOUND c SYSTEM i f f Q: DEMOD. SYSTEM 22 m. REC. I

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PARTIALLY FILTER F UNBALANCED w L" EC DEMODULATOR 22 v 3MHz SCI REFSIGNAL FROM so A /jfi\ freq. 1 2 3 3.58 E Ey MHz B O F 3MH-z F162 A C II 1 O M 3.58 5 6 ms E I i 1 358 INVENTOR. NORMAN W. PARKER B; 1mm, 4%,1" KW ATTORNEYS.

United States Patent 3,405,230 COLOR TELEVISIONDEMODULATION SYSTEMNorman W. Parker, Whe'atomtllL, assignor to Motorola,

Inc., Franklin Park, III., a corporation of Illinois Continuation-impartof'application Ser. No. 504,749,

Oct. 24, 1965. This'application Feb. 12, 1968, Ser." A No. 704,620 Y .12Claims. (Cl. 1785.4)

'QABSTIRACT' OF THE [DISCLOSURE I A direct demodulator for a compositecolor television signal operates upon both the video frequencybrightness components andthe chroma subcarrier. The demodulator outputis a signal representing brightness,-hue and saturation of atelevision-image. Modulation of the brightness carrier frequency,producing undesired spruious signals, is

offset by action of a cancelling network.

- Cross-reference This applicationis a continuation-in-part of myapplication Ser'. No. 504,749 filed Oct. 24, 1965.

' Background v trum use. The interference between these signals islimited because each has energy bunches which are interleaved in thespectrum in accordance. with known principles.

In many instances the chroma subcarrier is band selected andsynchronously demodulated atthree different "components with thedemodulator control signal of subphases toproduce color diiferencesignals (R'-Y, B Y

and G-Y) which are subsequently combined with the brightness signal (Y)to produce color representative'sig- "nals (R; B and G) for controllingthepicturetub'e: In

some color television systems the composite signal is applied directlyto a demodulator, so that a color representative signal is directlyproduced withouta matrixing operation, but such apparatus has not beensatisfactory because spurious signals'were developed and the televisionimage wasimpaired by an undesired pattern from these spurious signals.

An objecthereof is to directly demodulate color tele vision signals fromthe composite signal, while avoiding the production of spurious signalcomponents.

Another object is to improve a color television receive and render thereceiver more readily produced using semiconductor devices.

' A further object is to reduce the number of signal conductive channelsin a television receiver and to improve the correlation of the red, blueand green representative signals. s

' Summary The system hereof is a wide band direct demodulation systemfor acomposite color television signal which provides phase detection ofacolor subcarrier signal with the 7-,

,associated brightness signal. The subcarrier signal is modulated atvarious phase angles to represent saturation ofa givencolor'of theimage. Three such direct, primary demodulators produce, red,'blue andgreen representative signals including the, associated brightnessinformation so that the signals can be directly applied to a colorpicture tube. Since the brightness signal components extend in frequencyup to and generally into the frequency range of the subcarriermodulation band, these components modulate with the subcarrierdemodulation or reference signal to produce. a lower modulation sidebandof that signal which falls in the frequency range from the colorsubcarrier down into the color representative signals. These spurioussignalsare reduced or eliminated through a balancing or cancellingaction provided by a secondary demodulator which is controlled by asignal phase loclged .to the subcarrier and. modulated by the videofrequency brightness components. The output of this secondarydemodulator is applied in proper phase andamplitude to the primarydemodulator circuitry in order to cancel and offset the spurious signalsproduced therein. The complete demodulation system further includesprovision for modifying the ratio of the. brightness components to colorinformation components to compensate for selected ratios which may beused in transmission of the signal in order to promote compatibility ofthe brightness signal components for reproduction by monochrometelevision receivers. I 1

The drawing FIG. 1 is a-block diagram of a color television receiver inwhich the invention maybe used;

FIG. 2 is a series of frequency response curves useful in explainingcolor television signal'demodulation;

FIG, 3 is a block diagram of a demodulator useful in explainingdemodulation in a color television receiver;

FIG. 4 is a block diagram of a demodulation system illustrating theinvention;

FIG. 5 is a schematic diagram of a portion of the receiver of FIG. 1showing a specific form of the invention; and v FIG. 6 is a blockdiagram illustrating apparatus which may be substituted for a portion ofthe circuit of FIG. 5.

Embodiments The color television receiver of FIG. 1 includes tuner andIF amplifier stages 11 which provide a selected and amplified televisionsignal and apply it to the video desubcarrier to drive the loudspeaker'15.

The demodulated television signal from the video detector 12 is directcurrent coupled to an amplifier 17 and from there to the demodulationsystem 20 which provides separate red,blue and green representativesignals to-the respective amplifiers'22, 24 and 26. These amplifiers areindividually connected to the cathodes of the tri-beam cathode ray tube30 to individually drive the electron guns in this tube in accordancewith known operation in the art for production of a composite image incolor.

The image reproducer or color picture tube 30 includes a plurality ofcontrol grids which are interconnected to the arm of a potentiometer 33to provide a fixed bias for these grids, as a so-called masterbrightness, or beam current control for the tube 30.

The signal amplifier 17 is also coupled to an AGC system 40 whichprovides a control potential that is gated from and variable with'theamplitude of a received signal in order to adjust the amplification ofvarious stages in the circuitry 11 to maintaina relatively constantamplitude of the signal derived in the video detector 12. Amplifier 17also feeds the sweep or deflection circuitry 42 which is coupled to thedeflection yoke 44 to provide suitable sawtooth scanning currents todeflect the beams of the tribeam cathode ray tube 30 across its screenfor production of the image. The horizontal sweep circuit also generatesa suitable high voltage for the screen in the picture tube art, thesynchronizing pulses in the television signal, utilized to control thesweep circuitry 42, arealso accompanied by short bursts of referencecontrol signals at approximately 3.58 megacycles to be used forsynchronization of the oscillator source 46. Three different phases ofthe oscillator signal will be produced at the output terminals 48, 49and 50. For example, the signal at terminal 48 may be phased atapproximately 240 with respect to the blue color difference signal, thesignal at terminal 49 may be phased at approximately zero degrees andthe signal at terminal 50 may be phased at approximately 97 with respectto the blue color difference signal. The exact phase angles of thereference .signals at these terminals would be determined by severaldifferent variables within the receiver itself, such as the dominantcolor of emission of the various phosphors in the screen of the tube 30,even though the received television signal is a standard one of the NTSCtype.

The signal applied to input circuit 20A of the demodulator system isillustrated in the response vs. frequency curves of FIG. 2A. The videofrequency brightness components E extend from zero to over two mHz., andsometime as high as 3 or 4 mHz. The subcarrier wave representingchrominance or color difference information is modulated atapproximately 3.58 mHz. with a component of one phase E in a band ofapproximately .5 mHz. and in another phase as a vestigial sidebandsignal E with a lower sideband extending below 3 mHz. Present dayreceivers generally use a subcarrier bandwidth of approximately .5 mHz.total for all subcarrier components and the following description willassume such operation. However it will be apparent to those skilled inthe art that the principles discussed are equally applicable to derivingcolor components at other bandwidths.

It should further be noted that the frequency response characteristicsof the various parts of the receiver can be modified and correlated inaccordance with known receiver design inorder to establish an overalldesired video response. This correlation within the receiver need not bediscussed to understand the present invention.

The NTSC signal has video frequency brightness components in thefollowing relationship in order to improve compatibility for receptionby a monochrome receiver so that its grey scale of a color image will bein accord with the brightness response of the colors by the human eye.The brightness signal is as follows:

(1) E =0.30E +0.59E +0.11E

In this formula E represents a signal voltage for the luminancecomponent of a picture element and E E and E are respectively the signalvoltages representing the colors red, green and blue for that pictureelement. The chrominance modulated subcarrier wave is represented asfollows:

In terms of the green chrominance component the subcarrier can berepresented as:

( sc= a( o- Y) cos (wt-F146") +K sin (w!+146 In the above formulas Kequals 1/ 1.4; K equals 1/ 2.203; K, equals 1/0.70; and K equals As isunderstood in the television art the signals are constituted for propercompatibility for monochrome reception of the signal and production of agrey scale matchamplitudeswing of the quadrature modulated'subcarrierwave with respect to B in order to limit the size of the compositesignal (E =E +E for proper handling in a receiver. v p 7 FIG. 3represents a direct color'signal demodulator. The composite signalincluding the brightness components and the subcarrier waveis, appliedto the demodulator 201 which is controlled. in conduction by a referencesignal of the subcarrier frequency and properly phased with respectthereto. For example, the signal from terminal 50 can be used todemodulate for the red representative signal. The output of thedemodulator 201 is applied to a video frequency filter 202 to establisha low pass range up to, for example, 3 mHz. to be applied to theamplifier 2270f FIG. 1 in the case of a red representative signal.Aswill be explained in greater detail subsequently, the demodulator 201may be of the balanced type which is partially .unbalanced to compensatefor thebrightness to .subcarrierratio of the composite signal. Thisunbalance effectively compensates for demodulator efiiciency and the Kfactors in the equations of the signal.

An understanding of the functioning of the circuit of FIG. 3 can be hadby considering the reference signal as a rectangular gating signalcausing the demodulator to switch or sample the applied composite inputsignal by electron control means (e.g. a diode) which is essentiallyeither conductive or nonconductive. Mathematically, the operation can beexpressed by multiplying the composite color signal B by a cyclicfunction having a value 1 with the electron control means closed and thevalue zero when the control means is open. Such a gating signal todemodulate for red information can be represented as: v

(4) G(t) ='AT/T(1+2A cos tut-{2.4 cos 2w! 2A cos nwt) where and AT/ T isthe duty cycle For demodulating the blue signal the cosine terms becomesine terms and for the green signal, the cosine angle is w+146.

To develop the red representative signal the composite signal ismultiplied by the gating function and the product is as follows: I

In Equation 5 the first and fifth terms combine (by adjusting theunbalance of demodulator 201 to control E since K is less'than 1 and Acannot exceed 1) thus producing signal E and E of FIG. 2B. Similardemodulation can take place for the blue and green representativesignals with proper demodulator unbalance.

The second and third terms of the product in (5) are the originalsubcarrier components which are removed'to the extent they fall outsidethe passband of the filter 202 as seen by comparing the passband F ofFIG. 20 with the modulation range E of FIG. 2A, the wider rangechrominance modulation being ignored as previously discussed. The sixthand seventh terms of the demodulation product are at twice thesubcarrier frequency and higher so that they fall outside the videosignal bandpass F. The fourth term represents a modulation product whichis a spurious signal due to beating of the brightness signal E with thefirst order periodic component of the gating signal. Depending upon thefrequency range of the brightness signal applied to the demodulator 201(FIG. 3), the lowersideband of this spurious component S (FIG.'2E) canextend all the way from the frequency of the-refer'ence signal (3.58m/Hz.) down to zero, and therefore throughout most or all of the videopassband F. The higher the frequency of the brightness component applied to the demodulator the lower in frequency the lower sidebands willextend within the output range of filter 202. Such a spurious signal Smay have a substantial amplitude so that it appears as a pulse on theedge of a luminance change in the reproduced image. Since this spurioussignal will be changing in phase with others produced by the blue andgreen signal demodulators this undesired portion of the image willappear to move along the luminance difference transition in the picturegiving the appearance of a crawling pattern. Thus, the problem isproduced when the highest frequency of the brightness signal E is closeenough in frequency to that of the reference signal (here 3.58 mHz.)such that their modulation product, or part of it, falls within the lowpass range from the demodulator to the picture tube.

The low pass filter 202 will remove the upper sideband component of thefourth term of the modulation product and the carrier thereof, but thelower sideband remains. That portion of the brightness signal Einterleaved with the subcarrier E in FIG. 2A can be separated from thesub-carrier by known comb filter techniques. However, those signals maybe tolerable in some practical systems and they are not normally removedin the present day commercial receivers. In'accordance with teachingshereof the brightness components lower than the lowest selectedsubcarrier sidebands are prevented from generating cross colorinterference or spurious signal as described below. If comb filtertechniques are used to separate the brightness and chrominancecomponents the spurious signal elimination as described below can beused for the entire luminance range.

As shown in F164 the spurious signal S of FIG. 2E due to intermodulationof the brightness and subcarrier reference can be offset by the additionof another input E after which the combined signals are appliedto thedemodulator 201. Referring to FIG. 2, the reference signal of twice thesubcarrier frequency for the secondary modulator, approximately 7.16mHz.,,is modulated in 203 with the brightness signal E to produce lowerside- .band modulation components C,of FIG. 2D. Then this modulation ofthe second harmonic ofthe reference signal beats with the referencesignal in'demodulator 201 and that lower sideband is represented as C inFIG. 2E.

The upper sideband thereof is ignored since it falls outside of thepassband of filter 202. The energy spectrum C is seen to be equal andopposite in phase to the energy spectrum S of the spurious signal sothat these two off set one another in the demodulator 201. Thisoperation may be understood mathematically as follows:

COS Let-I-K2(E E sin wt+2A E COS (Diff-A 1K1(E E 1K1 v COS 2wt+0thercomponents-ZE cos 2A E cos wt) In Equation 6 the first through sixthterms are the same as those of Equation 5. The seventh term includes thecomponents in the seventh term of Equation 5 plus additional productsthat can be filtered or treated with the others specified. The eighthtermof Equation 6 is a product due to the addition of the cancellingsignal but it is -at a frequency beyond the video range of interest andcan be filtered in filter 202. The ninth term of Equation 6 is seen tobe equal and opposite to the spurious signal represented by the fourthterm of Equation 6 so that these two cancel one another. This operationis represented graphicaly by FIG. 2B in which the energy band Srepresents the spurious signal and the energy band C rep,- resents theoffsetting signal produced in the primary demodulator 201 when it is fedwith the signal from the secondary modulator 203.

In the. case of demodulators for the blue and green representativesignals, the demodulator unbalance would be suitably modified, thecoefficients are different and the phase is Zwt+146 for the greenrepresentative signal and sine 2w! for the blue representative signal.

In the specific circuitry of FIG. 5 the demodulated composite videosignal is coupled from the amplifier 17 to the base electrode of atransistor 60 in the phase splitter 62. DC bias for the base electrodeis provided by a voltage divider 63. Suitable output load impedances 64and 65 are connected respectively to the emitter and collectorelectrodes of the transistor 60. Opposite phases of the composite videosignal are coupled to the emitter follower stages and 72 which developthe signal across the emitter load resistor 73 and emitter load resistor74, respectively. The composite video signal of one phase is appliedfrom the load resistor 73 to the cathode of diode 75 and the videosignal of opposite phase is applied from a variable tap of resistor 77to the cathode of the diode 78. The anodes of diodes 75 and 78 indetector 20B are respectively connected to opposite terminals ofatransformer winding 80 which is coupled to a winding 80A. The winding80A is connected to the terminal 50 of the reference oscillator source46 to provide an effective switching voltage for the diodes 75 and 78 torender these diodes conductive during opposite phases of the referencesignal. The diodes 75 and 78 are connected in a balanced demodulatorcircuit with some amount of unbalance provided by the setting of avariable resistor 77.

Briefly the operation of circuitry associated with diodes 75 and 78 todemodulate the chrominance modulated subcarrier involves alternateconduction of the diodes 75 and 78 due to the reference oscillatorsignal from circuit 46, and these diodes alternately conduct oppositephases of the applied video signal. Since the reference oscillatorsignal applied through winding 80A has a particular fixed phase relationwith respect to the subcarrier frequency, the conduction of diodes 75and 78 represents the amplitude vairations of that particular phase ofthe subcarrier as the output signal is applied to the filter 82.

Output signals are derived from a tap of the winding 80 and coupledthrough the filter 82 to the red representative signal amplifier 22.Filter 82 includes a low pass section 82A having series inductors andshunt capacitors, and a further bridge-type low pass network 82B inorder to effectively define a bandwidth of Zero to 2 or 3 megacyclesfor-translating the red color representative signal and any highfrequency components extending out to the maximum range of luminancesignal being received. The filter 82 removes such signals as the 3.58(approx) reference signals applied from the oscillator 46.

It may also be seen that the luminance components of the composite videosignal are applied to the demodulator 20B. Normaly these luminancecomponents would be balanced out in causing equal and oppositeconduction of the diodes 75 and 78. However, conduction of the luminancecomponents by the diodes is made unequal by adjustment of variableresistor 77 so that a selected amplitude of the luminance components isnot balanced out in the circuit. Variable resistor 77 is set so that aprecise value of the luminance components exists for the amount of theluminance component associated with the demodulated chrominancecomponents resulting in a color representative signal to be translatedfrom the demodulator 20B.

In order to reduce or remove the brightness and reference signal productwhich is spurious, the input circuit 20A includes a secondary modulatorcircuit coupled between the output of amplifier 17 and the input to thephase splitter circuit 62. This signal path is effectively in shunt witha series input impedance comprising capacitor 120 and resistor 121.

The demodulated video signal at the output of amplifier 17 is applied tothe phase splitter stage 125 having a transistor 126 with collector andemitter electrodes providing'opposite phases of this video signal. Theoutput signals 'of phase splitter 125 are applied to the balanceddetector 130 which is controlled by a switching signal from thefrequency doubler 132. The doubler 132 is coupled to the terminal 50 ofthe reference oscillator source 46 so that a 7.16 (approx.) megacyclesignal is modulated by the B video signal in the circuit 130. Frequencydoubler 132 provides a phase locked signal of precisely twice thefrequency of the signal appearing at terminal 50. The output of thebalanced detector 130 is supplied through a 7.16 megacycle trap 135 tothe emitter follower 136 having a transistor 138. The emitter circuit oftransistor 138 is coupled through a filter network 140 to the base ofthe phase splitter transistor 60. Filter 140 defines a bandwidth whichselects the lower frequency sideband components of the 7.16 megacyclereference signal from about 3.7 me. to 7.16 mc.

Thus, the output of the spurious signal cancelling network 125, 130, 136and 140 is a range of sideband modulation components formed by theluminance signal and these beat with the reference carrier in detector20B to produce an equal and opposite energy curve C (FIG. 2B) ascompared to curve S in order to cancel the spurious signal. The phaseand frequency of the signal from doubler 132 adn the polarity of thediodes in demodulator 130 insure the proper cancelling relation.

Looking at the operation another way, the generated cancelling sidebandC together with the sidebands of curve S made by the original luminancecomponents conducted through elements 120 and 121, form a doublesideband signal in phase quadrature with the reference signal atterminal 50. Since the demodulator circuit 20B does not respond tosignals in quadrature with the reference signal applide thereto, thedescribed spurious luminance components, represented by curve S in FIG.2E, are eliminated in the output of the demodulation system.

In'the circuit of FIG. 5 a portion of the brightness video signal, E isavailable in the demodulator due to the unbalance of the circuit 72.This of course means that the brightness signal, E is available forproper demodulation with the subcarrier wave to directly produce a colorrepresentative signal. Accordingly control of variable resistor 77adjusts the level of the B signal with respect to the color subcarrierfor compensating the demodulator efiiciency and the coefficients of thecomposite video signal.

To derive correct information from the signal without unbalancing toadjust B the constants A should be 1.14 for red, 2.03 for blue and 0.70for green. Whereas the green representative signal can be demodulatedwithout special techniques, the red and blue representative signalsrequire special handling in order to establish the proper ratio ofbrightness signal to color representative signal to end up producing avideo signal representative of brightness, hue and saturation forapplication to the tri-beam picture tube 30. In a gating signal for abalanced demodulator, rather than one unbalanced to adjust E considerthe A constants to represent the coefficient of a Fourier expansion ofthe gating function. If the gate signal is assumed to be rectangular (asit may be for practical purposes if it has sufficient amplitude, eventhough it may be a sine wave) the coefficients can be represented as:

where F=the duty cycle of the gate pulse. Since this function has amaximum value of unity, a technique must be used to effectively achievea K over one in the signal.

Changing the K coefficients in the signal is done by adjusting theamplitude of the subcarrier wave with respect to the brightnesscomponents below the band of the subcarrier. FIG. 6 illustrates acircuit to achieve this in place of unbalancing the modulator 70 of FIG.5.

In FIG. 6 the amplifier 17 couples the composite brightness andsubcarrier signal to a bandpass filter which selects the carrier wave,and to a low pass filter 152. which passes the brightness component. Thefilters 150 and 152 have constant and equal time delays. The output offilter 150 is coupled to an amplifier 154 and from there to the addercircuit 156. The output of filter 152 is also coupled to resistive addercircuit 156 and to the base electrode of transistor 126 in the circuitof FIG. 5. Amplifier 154 pro vides sufficient gain to bump or step upthe subcarrier signal with respect to the brightness signal B that isdeveloped at terminal 160. Terminal 160 is connected to demodulator 20C.Terminal 162 is connected to transistor 60 and provides a step properfor the red representative signal. Terminal 164, being tapped downfurther on the output divider 165 of the adder circuit 156, provides astill further reduced step of subcarrier suitable for driving the greensignal demodulator. The fact of increased brightness signal level in therange of the subcarrier, that is above the lower cutoff of the filter150, is normally offset by the reduced response of the filter at theoutput of each demodulator (filter 202. or filter 82).

The circuit of FIG. 6 can be used to drive the demodulators 20B, 20C and20D of FIG. 1 each with the proper ratio of subcarrier Wave tobrightness signal so that the values of the K constant are effectivelyincreased to the point that the products with the A constants can be 1.The output of the low pass filter 152 could also provide the brightnesscomponents to each of the secondary modulators associated with thedemodulators 20B, 20C and 20D and corresponding to the circuitry 126,130, 136, 140 and 132 of FIG. 5 or to 203 in FIG. 4 (if 201 were fullybalanced).

The above described demodulation system directly produces colorrepresentative signals without the production of spurious signalsnormally associated with translation of the luminance components in ademodulator for the subcarrier wave. In this way the brightnesscomponents need not be separately translated at high level for matrixingor application to the color picture tube thus improving the receiver andpermitting the adjustment of simply the direct color representativesignals on the image reproducer during construction and alignment of thereceiver. The system is also operative with signals at intermediatefrequency if suitable filters are used to select the signal components.While it may appear at first glance that the overall demodulation systemincludes a great number of components, it will be recognized by thoseskilled in the art that certain modifications and simplifications can bemade within the teachings hereof and that the system lends itself toadvance solid state production techniques for practical use.

I claim:

1. A color television demodulation system having a circuit for supplyinga color television signal including video frequency brightnesscomponents in a given frequency range and a subcarrier wave modulated inamplitude and phase to represent color information, and an oscillatorcircuit providing a signal of the subcarrier frequency at a selectedphase for demodulating the subcarrier wave, said demodulation systemincluding in combination:

a demodulator circuit including electron control means coupled to theoscillator circuit to be controlled in conduction at the selected phase,

input circuit means applying the color television signal including thevideo frequency components and the subcarrier wave to said electroncontrol means to develop directly a video signal representingbrightness, hue and saturation and further developing a spurious signalwithin the given frequency range from modulation of the oscillatorsignal by the video frequency components, and

a spurious signal cancellation circuit coupled to said demodulatorcircuit and including further electron control means controlled by theoscillator circuit and the video frequency components to develop acancellation signal for the spurious signal.

2. The demodulation system of claim 1 further including means forestablishing a selected ratio of the amplitude of the video frequencybrightness components to the amplitude of the subcarrier wave tocompensate for the ratio thereof in said color television signal.

3. The combination of claim 2 in which said means for establishing aselected ratio is a partially balanced demodulator in said demodulatorcircuit.

4. The combination of claim 2 in which said means for establishing aselected ratio includes frequency selection means for the videofrequency components and fre quency selection means for the subcarrierwave and means for combining signals from both of said frequencyselection means with selected amplitudes and applying the same to saidinput circuit means.

5. The combination of claim 1 in which said electron control means arediodes and in which said demodulator circuit includes a low pass outputfilter for selecting the video signal from said demodulator system.

6. The demodulation system of claim 1 in which said spurious signalcancellation circuit is a secondary modulator for the video frequencybrightness components and a signal controlled by said oscillatorcircuit, said secondary modulator operative to produce the cancellationsignal in said demodulator circuit with a phase to render the spurioussignal ineffectual.

7. The demodulation system of claim 1 in which said spurious signalcancellation circuit is controlled by the oscillator circuit with asign-a1 at twice the subcarrier frequency.

8. The demodulation system of claim 7 in which said spurious signalcancellation circuit is coupled to said input circuit means to apply thecancellation signal to said demodulator circuit whereby cancellation ofthe spurious signal takes place therein.

9. A color television demodulation system for utilizing a compositesignal including video frequency brightness components and a subcarrierwave modulated in amplitude and phase to represent color information,said subcarrier wave having modulation components 'at least partiallyoverlapping in frequency the brightness components, said demodulationsystem including in combination:

a first synchronous demodulator including an input circuit for thecomposite signal and means for applying thereto a control signal of thesubcarrier frequency, said first demodulator also including an outputcircuit for the demodulated video signal representing brightness, hueand saturation information in the composite signal, the video frequencybrightness components beating in said first demodulator with the controlsignal of subcarrier frequency to produce a spurious signal in saidoutput circuit,

and a second synchronous demodulator including means for applyingthereto the video frequency brightness components and a signal phaselocked to the subcarrier frequency to beat the same together to producean offsetting signal for the spurious signal,

and means for applying the offsetting signal to said first synchronousdemodulator with an amplitude and phase to offset development of thespurious signal in said first synchronous demodulator.

10. A signal demodulation system, including in combination circuit meansfor providing a demodulated color television signal comprising videofrequency luminance components in a given frequency range and asubcarrier modulated in amplitude and phase to represent colordifference information and having modulation components overlapping thegiven frequency range, oscillator means providing three oscillatorsignals of the subcarrier frequency and at three difierent phases fordemodulating three phases of the modulated subcarrier, three demodulatorcircuits each connected to said oscillator means to be controlled by oneoscillator signal therefrom and each connected to said circuit means sothat each is supplied with the luminance components and the modulatedsubcarrier, variable means for adjusting the relative amplitude of theluminance components and the modulated subcarrier, said demodulatorcircuits eaoh detecting one phase of the modulated subcarrier andcombining the same with the luminance components, the luminancecomponents modulating the oscillator signals to produce spurious signalcomponents within the frequency range of the luminance components in thedemodulator circuit, and means responsive to the luminance components tocancel at least a portion of the spurious signal components, therebyproducing three different color representative signals.

11. The demodulation system of claim 10 wherein said means responsive tothe luminance components to cancel the spurious components includes afurther demodulator controlled by said oscillator means to producemodulation components for rendering at least one of said demodulatorcircuits unresponsive to modulating the luminance components and theoscillator signal.

12. A direct color signal demodulating system including in combination,circuit means providing a demodulated color television signal comprisingvideo frequency luminance components in a given frequency range and asubcarrier modulated in amplitude and phase to represent colordifference information and having components overlapping the givenfrequency range, oscillator means providing an oscillator signal of thesubcarrier frequency and a particular phase for demodulating one phaseof the modulated subcarrier, a demodulator circuit connected to saidoscillator means to be controlled by the oscillator signal therefrom andadapted to be connected to said circuit means to be supplied with theluminance components and the modulated subcarrier, said demodulatorcircuit being operative to produce a spurious frequency component withinthe frequency range of the luminance components in said demodulatorcircuit by modulation of the oscillator signal by the luminancefrequency components, means coupling said circuit means to saiddemodulator circuit to apply to said demodulator circuit the luminancecomponents and the modulated subcarrier modified in signal componentcontent to prevent formation of the spurious frequency component in saiddemodulator circuit, said demodulator circuit detecting one phase of themodulated subcarrier and combining the same with the luminancecomponents to produce a color representative signal.

ROBERT L. GRIFFIN, Primary Examiner. R. MURRAY, Assistant Examiner.

